A screw is widely used as a fastening member for fastening two fastened members to each other. For example, when a first fastened member and a second fastened member are fastened to each other, the screw is inserted into a screw insertion hole of the second fastened member from a side of the second fastened member facing away from the first fastened member, and screwed into the first fastened member. In an accelerator device disclosed in Patent Literature 1, a “tamper-proof screw” which is a type of the screw is used to fasten a housing and a cover to each other.
In Patent Literature 1, the screw is screwed in a state where the housing and the cover are combined together so that an edge of a screw insertion hole of the cover abuts against an edge of a screwing hole of the housing. For that reason, the edge of the screwing hole of the housing is compressed between the cover and a screw thread. As a result, a large tensile stress is generated in a root of a thread groove in the vicinity of the edge of the screwing hole whereby breakage such that the edge of the screwing hole is chipped may occur.